Various specialized equipment and apparatuses have been known to be provided for purposes of coating or granulating, which generally comprise a cylindrical partition disposed vertically in a container and a sprayer disposed concentric with the partition, the partition is suspended above a perforated air distribution plate lined with a metal mesh thereon, wherein the partition separates the container into a central upward flow bed area and a surrounding downward flow bed area. The air distribution plate is perforated with openings in a larger dimension in the central area inside the vertical projection area of the partition and with openings in a smaller dimension in the surrounding area outside the vertical projection area of the partition. An annular slit exists vertically between the partition and the air distribution plate and function as an annular passage for the particles flowing from the downward flow bed area to the upward flow bed area.
Due to the high velocity of a stronger upward air stream passing through the central area of the air distribution plate, a minimal static pressure is inwards created in the annular slit beneath the wall of the partition and in the annular border zone over the air distribution plate beneath the wall of the partition, meanwhile the particles are kept in a suspended semi-fluidized status by means of a weaker upward air stream passing through the surrounding area of the air distribution plate, wherein the weaker upward air stream creates a fluidized layer over the surrounding area of the air distribution plate and is fitly able to prevent adhesions of the particles to be treated in the fluidized layer and to keep those particles slightly in movement. Therefore, the particles to be treated presenting in the fluidized layer outside the partition will travel towards inside of the partition through the annular slit beneath the wall of the partition due to the minimal static pressure created by the stronger upward air stream.
The stronger upward air stream in high velocity passing through the air distribution plate carries the particles to travel upwardly in the partition, where the particles are coated or granulated with spray discharged from a nozzle of a sprayer. Then the particles encounter the weaker upward air stream in low velocity in the expansion area above the partition. When the velocity of the air stream is insufficient to support the particles, the particles fall under gravity into the downward flow bed area and reenter into the high velocity air stream in the partition under the suction effect presented in the annular slit and encounter the stronger upward air stream. In such a way, a circulation of coating or granulating in the upward flow bed area and drying in the downward flow bed area is accomplished. According to the prior arts, the stronger upward air stream is necessary to be optimized to improve the process and quality of the product.
In an improved modification of such an apparatus (see U.S. Pat. No. 6,773,747 B2) includes an air stream diverter for blowing the particles away from the nozzle during the spraying process, the air stream diverter comprises a sleeve extending around the sprayer and having a plurality of holes near the nozzle. The sleeve is operatively connected to a source of compressed air, which forces air to discharge radially outwards along the circumference through the holes in the sleeve. Once a diverting air stream encounters a stronger upward air stream passing through an air distribution plate, a diverting upward air stream directing radially outwards is generated, which allows more sufficient development of the spray pattern so as to increase the effective area of the spray zone, such that the diverting upward air stream pushes the particles away from the nozzle and prevents the particles from collapsing into the nozzle before passing through the spray zone, thereby the process allows higher spray rate while reducing over-wetting, uneven-wetting and agglomeration of the particles. However, due to the high radial velocity vector of the diverting air stream in the process, the particles may collide with the inner wall of the partition, which can cause a risk of damage to the coating film or granules.
A series of embodiments of this prior art apparatus (see U.S. Pat. Nos. 5,718,764 and 6,492,024 B1) comprise an air vortex generator for improving and facilitating the process, a cylindrical partition positioned vertically over a nozzle and concentrically with the nozzle, wherein the nozzle is mounted in a circular hole in an air distribution plate at the same level as said air distribution plate, whereby an annular slot is formed around the nozzle, wherein the air vortex generator is positioned beneath the air distribution plate and located inside a plenum base and comprises an air guiding wall positioned below the air distribution plate and fitted to the edge of the annular slot, wherein the air guiding wall defines a rotational symmetric space having downward expanding horizontal cross-section area and may typically be shaped as a downward expanding cone. Deflecting elements are constructed rotationally symmetrically at the lower part of the air guiding wall and each deflecting element may typically have a vertical section and a slanted section deflecting the air stream in the same direction, such that an air vortex is generated and passes through the annular slots around the nozzle, then the air vortex carries particles along a spiral upward path through the spray zone and the upward flow bed area in the partition. It has demonstrated that such controlled air vortex contributes to increased occurrence of contact of the particles with the discharged droplets due to the spiral upward movement while passing through the spray zone. However, due to the high velocity vector in the axial direction the air vortex impedes the sufficient development of the spray pattern and the particle flow pattern before entering of the particles into the spray zone, which can cause a risk of over-wetting, uneven-wetting or agglomeration.
Another improved modification of such an apparatus (see US2011/0315079 A1) includes another art of air vortex generator, which is relative to the conventional apparatus suitable for smaller particles and can obtain a more uniform particle-coating and minimize adhesions of particles in the process, wherein the air vortex generator shaped as a circular plate has a plurality of configuration slots which are open outwards and at an angle with regard to the vertical direction, the air vortex generator is mounted in the center of an air distribution plate beneath a cylindrical partition and is coaxial with a sprayer. As a result of the inclination of the grooves, upward raising air is diverted into a vortex, wherein the air vortex brings the particles into a spray zone in the partition to achieve coating. The air vortex improves the heat transfer in a two-phase flow as a result of a longer path at a distance in the axial flow direction. Due to the better heat transfer, it can be expected to obtain a more uniform particle-coating, reduced adhesions of particles and overcome the problem with coating smaller particles. However, in one aspect, since the tangential velocity and axial velocity of the air vortex are not controllable separately, the ratio of the tangential velocity and axial velocity is unchangeable, so that the air vortex is not able to meet concrete requirements of the specific process varying in terms of batch size, particle size and process property, etc., which means the air vortex can't be optimized to suit a specific process. In another aspect, the air vortex generated by the air vortex generator shaped as a circular plate is characterized with being stronger in the central area near the sprayer and being weaker in the surrounding area away from the sprayer, such that the air vortex impedes the development of the spray pattern and therefore interfere with the sufficient atomization of the droplets, which can cause a risk of over-wetting and uneven-wetting. In a further aspect, due to the immediate expansion of the air vortex after passing through the generator and the sudden acceleration of the particles on the generator, the air vortex impedes the development of the particle flow pattern and therefore interfere with the motion of the particles, which means the particles are subjected to an undesired irregular movement in the process.
It is obvious that the aerodynamic means contributing to the development of the spray pattern and the particle flow pattern can advantageously improve the process and the quality of the product. On the one hand, such improved aerodynamic means allows sufficient development of the spray pattern and the particle flow pattern. On the other hand, the optimized convection path of the product causes the particles to be uniformly sprayed, which ensures a maximum efficiency of spraying and a very stable operation while increasing the spray rate and produces high quality product with premium physical attributes. The type of aerodynamically atomized nozzles is commonly used, for example using a high-speed jet of air to disperse a liquid jet into small droplets in form of a cone-shaped cloud or spray pattern, preferably dual-fluids nozzle is utilized.